The use of molecular biological techniques to investigate the transmission of the parasite Toxoplasma gondii in commercial sheep flocks

Emma K. Morley - University of Salford

An important problem in agriculture…

Toxoplasma gondii is a single-cell parasite that infects almost all warm blooded animals including humans, domestic livestock, dolphins and birds. It is found worldwide and is often very prevalent - for example, on average one in three humans are infected. Toxoplasma is well known for its ability to cause abortion in humans but is also a major problem for sheep farmers accounting for approximately one third of all lamb losses. It may also play a role in cattle abortion although another closely related parasite, Neospora caninum, is the main agent in cattle. It has been known for a long time that the definitive host for Toxoplasma is the cat - that is, the host in which the parasite can complete its lifecycle and become infective. Besides being transmitted in the faeces of cats, the parasite can be transmitted by two other routes - eating raw meat from an infected animal and vertically from mother to offspring. This last route of transmission has usually been considered as rare. As sheep do not normally consume meat, the main route of transmission to sheep has always been thought to be due to cat faeces contaminating hay, feed and pastures. As a result most sheep farmers avoid keeping any contact with cats. In our studies we set out to investigate whether it was the cat or vertical transmission that is the most important route of transmission in sheep.

How do we measure parasite transmission from ewe to lamb…

We developed a protocol for detecting the parasite from lambs that have either been aborted or have been born healthy. This protocol uses a molecular biological technique similar to that used in forensic detection of DNA - it detects the parasites in tiny amounts of umbilical cord tissue (from the lamb) collected after lambing. So we can directly measure parasite transmission from ewe to lamb.

High levels of transmission from ewe to lamb…

Our initial study of 88 lambings showed that vertical transmission of the parasite was occurring in 61% of lambings - this was unexpectedly high (Duncanson and others, 2001, International Journal of Parasitology, 31, 1699). Although abortion occurred in many infected lambs, there were a significant number of healthy but infected lambs which could pass the parasite onto future generations. To substantiate these findings, we carried out a larger study on several farms and over several years. A total of 392 lambings were studied and essentially the same results were obtained - vertical transmission occurred in 69% of cases (Williams, Morley and others 2005, Parasitology,130, 301).

Some sheep families are highly infected and have high abortion rates…

To test the importance of vertical transmission compared with transmission from the cat, we decided to look at parasite transmission in families of sheep on the same farm. If the cat was spreading the parasites (in hay, pastures etc) we would expect all families on the same farm which share grazing and feed to have the same proportion of family members infected. On the otherhand, if vertical transmission were important then some families that were infected would pass the parasite down their generations and others that were not infected would remain parasite free. To test this, we looked at a pedigree flock where we could accurately establish family relationships and where there were different family lines kept on the same pastures and feed. To our great surprise, we found that there were marked differences between different families on the same farm (Morley and others, 2005, Parasitology , 131, 181). Some families passed on the parasite to 100% of their lambs while other families passed on no parasites at all. When we looked in more detail at the lambing history of these families over an 11 year period, we found that those families which passed on parasites at a high level had high frequencies of abortion (up to 48%!) while families with low or zero parasite transmission had few abortions (as low as 0%) over the period.

A farmer-friendly strategy for reducing sheep abortion…

These results strongly suggest that the parasite is transmitted vertically in sheep and suggests a strategy for reducing parasite infection and abortion. If farmers were able to identify highly infected and high aborting families, then selective breeding from uninfected or low aborting families could reduce overall abortion rates. More research is needed to confirm these strategies but this research potentially shows a simple, cost effective way for farmers to manage their flocks to reduce abortion.